The present invention relates to the activation of fluid bed oxidation catalysts. More particularly the present invention relates to the activiation of fluid bed oxidation catalysts useful in the preparation of maleic anhydride from 4-carbon atom hydrocarbons, including n-butane.
Oxidation catalysts containing the mixed oxides of vanadium and phosphorus have been utilized to produce maleic anhydride from 4-carbon atom hydrocarbons, such as n-butane. Methods have been investigated for activating, or increasing the catalytic activity of these catalysts. It is taught in the literature to "condition" vanadium phosphate-containing maleic anhydride catalysts under the flow of a low level of hydrocarbon in air, such as 0.2 volume percent to 2 volume percent hydrocarbon in air at temperatures of 300.degree. C. to 600.degree. C., as in U.S. Pat. No. 4,171,316.
U.S. Pat. No. 4,122,096 teaches conditioning of a dehydrated catalyst precursor with CO, H.sub.2 or H.sub.2 S in the absence of oxygen, at a temperature of from 300.degree. C. to 600.degree. C.
U.S. Pat. Nos. 4,178,298 and 4,181,628 discuss activating a mixed vanadium and phosphorus oxide catalyst at temperature of 300.degree. C. to 500.degree. C. by passing over the catalyst, a gaseous hydrocarbon component having 2 to 6 carbon atoms with the exclusion of molecular oxygen.
Attempting to "condition" a fluid bed catalyst with low levels of hydrocarbon in air under normal operating conditions has been found to have little beneficial effect. Further, it is impractical and nearly impossible to utilize an in-reactor activation method in a fluid bed reactor which comprises contacting the fluidized catalyst with a hydrocarbon at high temperature in the absence of molecular oxygen. Commercial fluid bed reactors do not contain means for heating the catalyst bed and gas stream to the temperatures required to achieve the desired activation utilizing oxygen-free hydrocarbon feeds. The requisite external heating mode of operation is not desired for fluid bed reactions, and indeed fluid bed processes are attractive for the reason that such external heating means are not required for normal operation.
Where it is desired to activate catalysts containing the mixed oxides of vanadium and phosphorus for the partial oxidation of n-butane to form maleic anhydride, the activation of the catalyst with the hydrocarbon n-butane in the absence of oxygen would be extremely expensive, with regard to the volume of butane required to "fluidize" the catalyst bed. Further, stringent safety precautions would also be required to insure that no explosive mixtures of air/butane result outside the reactor.
Aside from economic and mechanical considerations, it is thought that contacting the vanadium phosphorus mixed oxide catalyst with reducing gases, including hydrocarbons, in the absence of oxygen merely causes reduction of the catalyst components (particularly vanadium) by extracting lattice oxygen, causing the catalyst crystallite structure to reach a static configuration. Oxygen atoms are thus depleted from the catalytic active sites, and are unavailable for reaction with hydrocarbon reactants to yield useful product. While this procedure may enhance the activity of catalysts that were initially over-oxidized, the overall effect upon catalysts having a proper component valence range could be detrimental over time by inducing change to the static configuration.